1. Field of the Invention
The present invention relates to a process for deactivating and recovering boron trifluoride in the preparation of polyisobutenes by cationic polymerization of isobutene or isobutene-containing hydrocarbon streams in the liquid phase in the presence of boron trifluoride as such or in the form of a boron trifluoride catalyst complex, in which the catalyst complex is separated as a substantially liquid phase from the discharge from the reactor.
2. Description of the Background
High molecular weight polyisobutenes having weight average molecular weights (Mw) of up to several hundred thousand Dalton have long been known, and their preparation is described, for example, in H. Guterbock: Polyisobutylen und Mischpolymerisate, Springer Verlag, Berlin 1959 pages 77 to 104. The highly reactive polyisobutenes which as a rule have weight average molecular weights of from 500 to 50000 Dalton and a high content of terminal double bonds, i.e. vinylidene groups, preferably substantially more than 60 mol %, should be distinguished from these conventional polyisobutenes.
Such highly reactive polyisobutenes are used as intermediates for the preparation of additives for lubricants and fuels, as described, for example, in DE-A 27 02 604. For the preparation of these additives, polyisobutene/maleic anhydride adducts, in particular polyisobutenylsuccinic anhydrides, are first produced by reacting the terminal double bonds of the polyisobutene with maleic anhydride and are then reacted with specific amines to give the final additive. The proportion of terminal vinylidene groups in the molecule is one of the most important quality criteria for this polyisobutene type since, in the adduct formation with maleic anhydride, mainly the terminal vinylidene groups react whilst, depending on their position in the macromolecule, the double bonds present further toward the interior of the macromolecules do not react or react to a substantially smaller extent, without the addition of suitable activators.
The generation of the terminal vinylidene groups and the isomerization of the terminal double bonds in the isobutene macromolecules to give internal double bonds are described, for example, in the article by Puskas et al., J. Polymer Sci.: Symposium No. 56, 191 (1976) or EP-A 628 575. The protonations, deprotonations and rearrangement reactions taking place there are equilibrium reactions in which the formation of more highly alkyl-substituted cations is thermodynamically favored. Said reactions are as a rule promoted by traces of acid, in particular by the catalyst of the polymerization itself, which is usually a Lewis acid catalyst.
A further quality criterion for polyisobutenes having the said intended use is their number average molecular weight (Mn). Number average molecular weight is a quantity which indicates the average molecular size present in the product of the polymerization. In general, polyisobutenes having a number average molecular weight of from 200 to 50000, preferably from 200 to 5000, in particular from 500 to 3000 and especially from 500 to 2500, Dalton are used.
The molecular weight distribution (dispersity, D) of the polyisobutene macromolecules is also a quality criterion for said purpose since, the broader it is, i.e. the greater the scatter of the molecular weights of the polyisobutene macromolecules about a mean value, often the less tailored are the products to a specific property.
A person skilled in the art is familiar with a number of processes for the preparation of highly reactive polyisobutenes from isobutene which have number average molecular weights and dispersities which meet said requirements and for which boron trifluoride is used as a catalyst.
Boron trifluoride is used predominantly in the form of donor complexes, in particular with water, alcohols, phenols, carboxylic acids, carboxylic anhydrides, hydrogen fluoride, ethers or mixtures of these compounds. Boron trifluoride, as such or in the form of said complexes, is a catalyst which is extremely effective even at low temperatures (cf. for example DE-A 27 02 604, EP-A 145 235 or EP-A 322 241).
If it is therefore intended to stop the boron trifluoride-catalyzed polymerization of isobutene after a defined conversion and/or a defined selectivity with respect to the macromolecular products has been reached, the boron trifluoride must as a rule be rapidly and completely deactivated. This deactivation may consist in decomposing the boron trifluoride, for example in hydrolyzing it with sodium hydroxide solution, or in complexing it with stronger donors in order to remove it from the reaction.
DE-C 40 33 196 states that the reaction can be stopped with ammonia or with from 5 to 50% strength by weight aqueous sodium hydroxide solution. However, sodium or ammonium salts which form thereby cannot be completely separated off from the reaction product polyisobutene, even by washing several times with water, and present problems in the applications described above, generally even in amounts of less than 10, often of even less than 0.1 ppm.
According to DE-A 43 06 384, the deactivation of the boron trifluoride can be carried out using water, alcohols, acetonitrile, ammonia or aqueous solutions of mineral bases, such as alkali metal and alkaline earth metal hydroxide solutions, or with solutions of carbonates of these metals.
The hydrolytic processes under aqueous conditions for deactivating the boron trifluoride all lead to waste waters which are problematic owing to their content of inorganic fluoride of course, the boron trifluoride also cannot be recycled economically for re-use in the process by this method. Since the processes of this type for the preparation of polyisobutenes generally have to be carried out at low temperatures in order to be sufficiently selective, the aqueous hydrolysis of the reactor discharge usually has to be carried out with heated water in order to be sufficiently rapid and complete and to avoid the formation of ice in the discharge. In these generally short heating-up phases, however, undesirable by-products may form, i.e. the overall selectivity of the reaction decreases. Particularly for industrial processes, however, this procedure also means that some of the energy consumed for reaching the low reaction temperature is lost. With the use of water, a larger or smaller amount of corrosive hydrofluoric acid is also virtually always formed, necessitating the use of high-quality and hence usually expensive materials, in particular special steels for the construction of the downstream parts of the plant.
The deactivation of the boron trifluoride with the aprotic acetonitrile (cf. for example EP-A 145 235) takes place rapidly. However, the toxic acetonitrile is generally used in excess and is readily water-soluble, so that large amounts of problematic waste water result during working-up.
WO-A 99/31151 discloses that boron trifluoride can be separated from the reaction mixture of the isobutene polymerization in the form of an isopropanol complex, and the boron trifluoride can be made available again for the reaction in this way. In order for this separation to take place, not more than 2% by weight of isobutene may be present in the reaction mixture. In practice, however, this content leads to products of reduced reactivity in the reaction of isobutene to give polyisobutenes having number average molecular weights of more than 1000, so that low isobutene contents are preferably avoided in this way and in these cases unconsumed isobutene is removed from the reaction mixture at the end of the reaction with considerable additional technical effort.